Heavy Rain: The Water Cycle and Climate Change

The global hydrologic cycle is a fundamental process that drives the movement of water in different states of matter throughout the planet, and is a salient indicator when monitoring the effects of climate change. Minor changes in the climate can result in massive changes to the water cycle, and as the planet gets warmer through the process of climate change, there is a lot that we can expect to happen and a lot that is already happening atmospherically and meteorologically that should be alarming. Rising temperatures are driving up rates of evaporation, afflicting areas of the planet with more intense droughts. With more water vapor being stored in the atmosphere, normal precipitation events become scarce and extreme precipitation events become more common, leading to increased surface runoff and floods.

Applying the Clausius-Clapeyron relationship, the capacity of the air to hold water vapor increases by 7% for each 1oC increase in temperature. (Trenberth, 2011). This fundamental principle of atmospheric science can offer an explanation for the recent increase in stronger hurricanes. The increased moisture in the air by way of this relationship increases the moisture available for the storm, with “more latent heating, a potentially stronger storm, with more convection and stronger updrafts, resulting in even more rainfall” (Mann, Peterson, & Hassol, 2017). The increased atmospheric temperatures resulting in increased evaporation will lead to more intense drought events.

The Palmer Drought Severity Index (PDSI), an indicative metric of drought, measures soil moisture and is depicted below for a 12 year span across the planet. A more negative PDSI indicates a more severe drought, and equatorial tropical areas seem to be affected the most, which makes sense as they are the areas of the planet hit by the most solar radiation.

(Pachauri & Reisinger, 2007)

Because the vapor is now more capable of holding moisture, precipitation events occur less frequently, and when they do they are much more intense. Drought following intense bouts of evaporation tend to affect water-stressed areas the most, particularly northern Africa, the Mediterranean, the Middle East, Northern China, Australia, parts of the USA and Mexico, Brazil, and western South America. Water basins in these regions have a water availability of below 1000 m3 per capita (Bates, Kundzewicz, Wu, & Palutikof, 2008). These regions also appear to be affected severely by drought, and have seen their PDSI drop over time as seen in the map above. As precipitation occurs less frequently and more erratically, these regions where frequent rainfall is vital for water security are affected severely by climate change.

It is hard to directly link climate change with the observed changes in precipitation patterns as there is some uncertainty related to natural variability in the climate. General trends among different sets of climactic data can be analyzed however to provide some insight on how the change in frequency and intensity of precipitation events can be associated with climate warming. The warming of the planet has seen more water stored as vapor in the air, and precipitation totals have decreased. However, there has been an observable increase in anomalous heavy precipitation events. (Bates, Kundzewicz, Wu, & Palutikof, 2008).

The general scientific consensus fails to reject anthropogenic activity as adding to extreme precipitation events, running the risk of a Type II error. However, data shows the latent heat of condensation of atmospheric water vapor for mean precipitation as being suppressed by human greenhouse gas emissions. Outgoing longwave radiation, naturally occurring radiation from the Earth’s surface is associated with normal precipitation events (Liebmann, et al., 1997), and greenhouse gases in the atmosphere hinder the emissions of this radiation released in the form of latent heat of condensation. Therefore a correlation can be made with greenhouse gas emissions of anthropogenic origin, and a decrease in normal precipitation events and a resultant increase in more extreme rainfall events in the mid latitude regions.

The warming of the Earth raises many other concerns with the increased volume of runoff and streamflow along with an increased frequency of floods. Factors that determine the occurrence and intensity of a flood include precipitation intensity, volume, timing, drainage basins, and the phase of the precipitation. Colder regions where snowfall is more frequent have and will continue to see melting of snowpack with rising temperatures, resulting in a proportionally higher level of rainfall. This results in an earlier peak streamflow, seeing an annual increase of 5% from 1935-1999 in the Arctic Drainage Basin (Bates, Kundzewicz, Wu, & Palutikof, 2008). Recently, in the basins of some Arctic rivers in Russia, there has been a 0.5-1% increase in catastrophic floods associated with river ice melting and extreme rainfall events. In the winter these effects were amplified as snowmelt and the thawing of permafrost added to the base streamflow.

Projecting the future of the hydrologic cycle with continued climate warming is prone to many uncertainties, including the inherent variability of climate, the uncertainty of future greenhouse gas emissions rates, and the uncertainty of global climate and hydrologic models (Bates, Kundzewicz, Wu, & Palutikof, 2008). “Where uncertainty is assessed qualitatively, it is characterized by providing a relative sense of the amount and quality of evidence and the degree of agreement” (Pachauri & Reisinger, 2007). Even if there are some natural forces driving some changes in the climate we are seeing, we do know that there is an anthropogenic contribution to the warming of the climate leading to changes in the global hydrologic cycle. We have seen policy succeed with the Montreal Protocol in 1989, banning chlorofluorocarbons (CFCs), an ozone-depleting substance (ODS) that chemically reacts with the oxygen in ozone, breaking it down and allowing for more solar radiation to reach the Earth’s surface. We saw large ozone holes form over the Antarctic and Arctic, and the policy enacted managed to bring about significant healing in the Antarctic ozone hole. Nonetheless we still have seen an increase in the emissions of some other ODS that have accounted for 14% of the radiative forcing from other greenhouse gases already mixed in the atmosphere (Estrada, Perron, & Martínez-López, 2013).

Policy and a paradigm shift in the global mindset towards climate change should lead the impetus to make improvement based on what we do understand and can predict. The current issue is that climate change itself is an issue up for partisan debate, as the fossil fuel industry continues to fund politicians to keep their interests and profits alive. Another concern is that developed nations, often the ones with the larger carbon footprints, are more insulated from the changes the climate undergoes and so it’s not necessarily an issue of concern for many people, as they don’t see the damage happening to them firsthand. Since we still have many uncertainties in predicting the effects of climate change, our efforts to combat it should preemptive and not reactionary to a cataclysmic event.